Neuropsychological deficits are among the main symptoms of obstructive sleep apnea syndrome (OSAS), which could be related to impaired cerebral blood flow (CBF). We conducted a study in 20 subjects tested in our Sleep Laboratory, to assess regional CBF and cognitive function in OSAS. Our measurements included technetium-99m hexamethylamino propylenamine oxime brain perfusion scintigraphy in wakeful state, i.e. in the morning after polysomnography and also cognitive function tests after polysomnography, in 20 patients, 16 male and 4 female, aging between 30 and 60 years. We found that apnea-hypopnea index was greater than or equal to 5 in 16 (85%) subjects, consistent with OSAS. Mean arterial oxygen saturation during sleep was correlated with CBF in all regions. Cognitive function test scores in verbal memory were positively correlated with percentage of sleep duration with less than 90% of oxygen saturation during sleep. Cerebral blood flow was not uniform in OSAS patients, and was significantly lower in the left frontal and left temporal regions as compared to that of these regions on the right hemisphere. In conclusion, our findings indicated association of CBF and verbal memory with hypoxemia during sleep and decreased perfusion after apneic episodes in the left frontal and temporal lobes in OSAS patients, which could also indicate impairment of upper airway motor control.